Radiotherapy is a technique by which high-energy x-ray beams are directed towards a tumour or other lesion within a patient. These beams pass through the patient's anatomy and cause harm to the tissue making up the tumour. The beam is however also apt to cause damage to healthy tissue around the tumour, albeit at a lower rate, and thus efforts are made to limit this potential for damage.
One way of doing so is to direct the beam toward the tumour from a range of different directions. The result is that whereas the tumour (or some part of it) is in the beam continuously, the surrounding healthy tissue is only exposed for a shorter period, thus reducing the dose applied to that specific region. Typically, radiotherapy sources are therefore mounted on a rotatable gantry, directed toward the axis of rotation. The patient is then placed so that the tumour is at or near that axis, and the gantry is rotated continuously around the patient so that the tumour is irradiated from all possible directions.
Another way of limiting the dose applied to healthy tissue is to collimate the beam, i.e. place blocking elements in the beam path so as to limit its cross-sectional area. This can be used in a number of ways, either to limit the beam shape to the shape of the tumour (when viewed along the instantaneous direction of the beam) to produce a “rotational conformal arc therapy”, or in more complex ways such as “intensity modulated arc therapy” which allows the gantry to rotate around the patient while the collimator shape and the dose rate are varied to build up a prescribed three-dimensional dose distribution in the patient. A discussion of these techniques is given in our earlier patent application WO2007/124760.
Such collimating techniques do however require a collimator that can define a variable and (typically) non-rectangular shape for the beam. Such a device is known as a “multi-leaf collimator”, and an example is shown in EP0314214. It includes a number of adjacent “leaves”, each of which is relatively deep in the beam direction (typically 10-15 cm) so as to provide the necessary attenuation of the beam, long in one dimension transverse to the beam (typically 30 cm or more), and narrow in their third dimension (typically a few mm). Arranged side-by side, each individual leaf can be driven forward and back along its long dimension so that the group of leaves as a whole defines an edge whose shape is variable at will. Typically, the leaves are supported along their long edges, located in guides having a corresponding shape. EP 0314214 shows the leaves being supported on rotatable wheels. They are driven by an electric motor positioned behind the rearmost edge of the leaf (to keep it out of the radiation field), driving a threaded rod that engages with a captive nut held in the leaf. As the rod rotates, the leaf is driven forward or back.